Joseph Ko Seong-un, also called ‘Seong-il`, was born in Byeolam, Deoksan, Chungcheong-do (now, Sangjang-ri, Godeok-myeon, Yesan-gun, Chungnam). He learned about the Catholic religion from his parents and practiced it with devotion.

By nature, he was very gentle and was loved by many people. He was a devout Catholic, as well as an affectionate and dutiful son to his parents. When his father was sick in bed for eight months, he prayed fervently with his elder brother Peter Ko Seong-dae for the recovery of their father`s health. The two brothers read the Bible together and meditated on it, and earnestly exhorted people to believe in God. In this way they became a good example to other believers.

His elder brother Peter Ko Seong-dae moved to Gosan, Jeolla-do and he was arrested there by the police of Jeonju during the Shinyu Persecution of 1801. At that time, he was tempted to save his own life, so he betrayed his religion and was set free. But on returning home, he immediately repented his mistake, and practiced his religion again with zeal, together with his younger brother.

Joseph Ko then moved to Noraesan, Cheongsong (now, Norae 2-dong, Andeok-myeon, Cheongsong-gun, Gyeongbuk) with his elder brother. He led a relatively peaceful life for a while with the believers of that village.
On Easter Sunday in 1815, while they were celebrating the Solemnity of the Resurrection of Christ, he and his elder brother Peter and all the Catholics of the village were arrested by the police who raided their place of worship accompanied by an informer, and were taken to Gyeongju. It was around February 22. This was the beginning of the Eulhae Persecution.

The Catholics of the Noraesan Christian Village thought the police were thieves and fought against them, under the command of Joseph Ko, who was a nimble and strong man. On realizing who the invaders were, they stopped fighting and surrendered themselves. Joseph Ko was the first to be arrested.

Sent to Gyeongju, Joseph Ko kept firm in his faith, by resisting the severe interrogations and torture, like his elder brother. Then the chief official of Gyeongju sent the two brothers, along with other believers who refused to renounce their faith, to Daegu which was ruled by the governor. In Daegu, they underwent interrogations and torture. They were imprisoned for more than seventeen months. Joseph Ko professed his faith in God by enduring all sorts of trials and hardships. He was sentenced to death.

On December 19, 1816 (November 1, by the Lunar calendar) he was taken to the execution ground in Daegu and beheaded with his elder brother Peter Ko Seong-dae and died a martyr. He was single and celibate.
Prior to their martyrdom, the governor of Daegu, who noticed that the two brothers remained firm in their faith, despite severe interrogations and torture, reported to the royal court as follows:

"The two brothers, Ko Seong-dae and Ko Seong-un, steeped in Catholicism, are so foolish and ignorant that they never realized their wrong-doing. I tried to help them to understand their wrong-doing in believing in the Catholic religion, but they never changed their minds. I cannot forgive their sins because they obstinately persist in keeping their resolution to die for their faith."

After he was martyred, Joseph Ko`s body was buried near the execution ground. Then on March 2 of the following year, relatives and believers took his body and reburied it in an appropriate place.